1. Field of the Invention
The present invention relates in general to a method for making crystallized glasses, and relates in particular to a method for making crystallized glass products from waste ashes produced from incinerating sludges and industrial and public wastes.
2. Technical Background
with increasing concern for preservation of environment, there is an increasing recognition for the importance of reducing and recycling waste materials. Effective utilization of sludges produced from sewage and industrial water treatment facilities is also an important aspect of reducing the need for landfill and preserving the natural environment.
The present invention relates to an apparatus for producing products similar to such materials as pebbles, stones, and crushed stones, and to an apparatus for producing such manufactured products as tiles and blocks, using the method for producing crystallized glasses which has been disclosed earlier in a Japanese Patent Application, First Publication, H2-413772.
There are conventional furnaces for melting waste ash materials produced from incinerating sewage sludge and other types of wastes, such as rotary furnaces, coke beds (or vertical shaft furnaces), surface melting furnaces, arc furnaces, plasma furnaces and microwave melting furnaces. However, the slags produced from these conventional melting furnaces are glassy types which are only useful as low grade construction materials such as road fill. There has been proposals to slowly cool or treat the melted materials to produce crystallized aggregates. The crystallized aggregates made by such methods (Japanese Patent Application, First Publication, S56-54248, Japanese Patent Application, First Publication S56-54247) are used as concrete admixtures.
The crystallized glass products made by the method of the present invention are far superior to those made by the conventional methods mentioned above, and the details of this method has already been disclosed in a Japanese Patent Application, First Publication H2-413772.
The building materials using crystallized glass are made by two approaches: one approach is to melt a feed material of an appropriately adjusted composition in a conventional glass furnace and after making formed glass, heat treating and crystallizing steps are performed on the formed glass; the other approach is to prepare particulate shaped glass first from a melt, and shaped glass is made by heat treating and crystallizing step is performed on the shaped glass. However, there has been no proposal for an apparatus using waste materials for feed and making crystallized glass products tailored to the end applications.
Applications of crystallized glass products for construction and building purposes are broadly divided into two categories: one is a group of irregular shaped products such as pebbles and crushed stones; and the other is a group of shaped products such as tiles and blocks. The irregular shaped products can be used in their original condition or used as raw material for secondary products as in the case of foundation stones or aggregates for terrazzo tiles. In either case, the end applications must be compatible with the characteristics of the crystallized glass, and used in applications having relatively high added value. For formed glass product applications, tiles and blocks can be used in interior applications such as decorative blocks and boundary stones, which are high-value-added products but correspondingly, the quality demands are also high. Therefore, it becomes important that an apparatus for making crystallized glass be constructed in line with the economic requirements, such as capital and operating costs, of the end products.
In considering the equipment for making crystallized glasses and products, the essential components are melting furnace, shaping apparatus and crystallization furnace.
With respect to the choice of the melting furnace, it is considered that the conventional glass melting furnaces are unsuitable because of the following three reasons. First, waste materials as feed to the melting furnace are quite variable in size and quality, they can vary from waste containing organic substances to nearly inorganic small sized particles of several tens of micrometers such as fly ashes from incinerators. Second, the ash compositions produced from the waste materials are multi-component, and the melt is highly corrosive to the refractories of the melting furnace. Third, it is necessary to satisfy production conditions for producing iron sulfide as a nucleating agent from iron oxide and sulfur compounds.
With respect to making irregular shaped crystallized glass products in the shapes of sand, particles, crushed stones and small lumps, manufacturing cost must be low because they have low added values, and it is necessary to shorten the processing time. Especially for the crystallization step, roller hearth kilns or tunnel kilns used as crystallization furnace for formed blocks, bricks and tiles are unsuitable because of their long processing times, and the large size of the equipment. They are also heavy consumers of fuel, thus increasing the operating cost.
Other problems with making the crystallized glass products are associated with the properties of the melt made from waste ash materials. Ordinary glasses exhibit good forming and malleability even at temperatures as low as 700.degree. to 800.degree. C., but the melt has a tendency to harden below about 900.degree. C. If there is a large temperature difference existing between the surface and the interior when making crystallized glass products, cracking may appear during the glass formation stage, or if large internal stresses are left in the glass, it may lead to cracking during the crystallization step. Therefore, it is necessary to control the cooling rate during the glass formation stage, and the conventional glass forming methods such as roll-drawing cannot be applied, because of the lack of thermal shock resistance of the formed glass and the difficulty of temperature control. Further, the method of melting and adhering small glass particles during the crystallization step is difficult to be used because the recycled glass is difficult to be softened before the crystallization temperature is reached.